A new wave of polling shows virtually every close Senate race growing even more competitive, raising the pressure on both parties in the final days of the midterm campaign.

In several states, Democratic candidates are on the upswing. Pennsylvania Rep. Joe Sestak, after trailing Republican Pat Toomey for months, took slim, 1- and 3-point leads in a pair of surveys released this week.

In Colorado, Democratic Sen. Michael Bennet cut Republican prosecutor Ken Buck’s advantage to a 3-point gap in a Reuters/Ipsos poll that placed Buck 9 points up as recently as August. In Wisconsin, a survey from Wisconsin Public Radio and St. Norbert College placed Democratic Sen. Russ Feingold closing to within 2 points of plastics executive Ron Johnson, who has held high, single-digit leads in most other recent polls.

That polling gives Democrats hope that they could hold their Senate defeats to a bare minimum, retaining most seats beyond the all-but-lost GOP targets of North Dakota, Indiana and Arkansas.

But for almost every positive result the majority party has gotten, other polls have shown Republicans gaining ground elsewhere – and reviving the prospect that the party could flip control of the upper chamber.

In Washington and California, the GOP’s two West Coast takeover prospects, Democratic candidates slipped into virtual ties with Republican challengers they led as recently as September. California Sen. Barbara Boxer was just 1 point ahead of former Hewlett-Packard CEO Carly Fiorina in a Reuters/Ipsos poll released Friday.

And Washington Sen. Patty Murray, who had 8- and 13-point leads in two state polls released last week, was within the margin of error against Republican Dino Rossi in a pair of new surveys. A poll from Marist College and the McClatchy newspaper chain had her at 48 percent to Rossi’s 47 percent, while PPP had the race at a 2-point margin.

Even before these new polls, three other campaigns were already locked in the political equivalent of trench warfare: The Illinois and West Virginia open-seat races, and the Nevada contest for Senate Majority Leader Harry Reid’s seat. No candidate has been able to take a strong advantage in any of those races, dating back to the start of the general election campaign.

Add up all those states and you have eight races that could make the difference between the majority party taking a superficial wound and hemorrhaging enough seats to lose power in the Senate.

The close race to the finish isn’t unique to this cycle, Ipsos pollster Chris Jackson said, but “whenever people start to really tune in, you see a lot of these races start to tighten up.”

“Back in August I think we saw a lot of Republican excitement, so these Republican candidates had huge leads,” Jackson explained. “This year, we’re seeing this tightening effect making Democrats look better because Republicans generally had the big advantage, early.”

But Democrats have trumpeted the newest polls as a sign that their party is successfully going on the attack against Republicans, waking up base voters – including young people, minorities and union members – to the stakes of the 2010 campaign. The AFL-CIO released a memo Monday morning taking credit for moving Pennsylvania union voters 23 points in Sestak’s direction and helping Manchin take a 40-point lead among the West Virginia labor community.

“There was a Tweedledee, Tweedledum attitude toward the parties,” said Mike Podhorzer, the AFL-CIO’s deputy political director. “When Toomey had his huge lead, it was definitely before voters knew how extreme his views were on outsourcing, Social Security.”

That’s no longer the case, he said, and “it’s changed the dynamics of the race.”

The Democratic Senatorial Campaign Committee and its candidates have moved to take advantage of the perception that the polls are moving their way. The DSCC leaked an internal survey showing Kentucky Attorney General Jack Conway seizing a two-point lead over Republican ophthalmologist Rand Paul. In Missouri, Democratic Secretary of State Robin Carnahan’s campaign commissioned a PPP poll that placed her only five points down against Rep. Roy Blunt.

The GOP isn’t buying it. Whatever the latest individual polls say, Republican candidates have held durable advantages in states like Wisconsin and Kentucky for almost the whole general-election campaign. While some races – like the Sestak-Toomey matchup – have gotten much closer, it’s been thanks to millions of dollars in Democratic spending that can’t be matched in every state. Some underdog Democrats – like North Carolina’s Elaine Marshall and Louisiana’s Charlie Melancon – may tick up a few points here and there, but it’s hard to see how they would make up the rest of the gap before Nov. 2.

And if the 2006 and 2008 cycles are any guide, the races that end up balanced evenly on Election Day tend to fall toward the party on the upswing.

Republican strategist Brian Nick, who was the National Republican Senatorial Committee’s spokesman in 2006, spelled out that thinking: “I find it very hard to believe that, the last few days of an election, the undecideds are going to decide, ‘You know, I actually like the direction the country’s going in right now. I’m going to vote Democrat.’”

“We saw that in ’06,” he recalled. “If a state was very close, a state like Virginia, a state like Montana, obviously went to the Democrats.”

So while Democrats may be succeeding at driving up the negative ratings for Republican challengers, incumbents like Bennet and Feingold, along with well-known open-seat candidates like Manchin, could have a hard time making it all the way to the 50-percent mark.

“Michael Bennet has hit a ceiling and can’t go above it,” insisted Colorado Republican Party Chairman Dick Wadhams. “Has it shown it’s tightened, the last few weeks? Apparently those national polls show that, but Michael Bennet hasn’t gone anywhere and he’s the incumbent U.S. senator.”

If that dynamic holds across the map, it could bring Republican gains well beyond a five- or-six-seat pickup, to the brink of a Senate majority that was unimaginable at the start of the cycle.

“Each one of these beasts is its own beast,” Jackson, the Ipsos pollster, said of the Senate campaign. But, he added: “There’s nothing that’s really indicated that this would be atypical.”